A gas sensor with a laser as the light source is known, for example, from U.S. Pat. No. 5,339,155 A, in which a gas-measuring arrangement with an open measuring section (“open-path sensor”) is described, in which the laser light is directed via a semitransparent mirror and an obliquely positioned mirror onto a concave mirror and from there as a parallel light beam onto a reflector located at a remote location. The laser light is reflected by the reflector and the measuring section is passed through again, after which the reflected laser light falls again on the concave mirror, which focuses the reflected light onto the obliquely positioned mirror. The reflected laser light now travels from the obliquely positioned mirror to the laser light coupled in originally.
Part of the reflected laser light is then cast by the semitransparent mirror onto a detector, while another part is lost.
This arrangement of concave mirror, obliquely positioned decoupling mirror in the focal point of the concave mirror and detector corresponds, in principle, to the design of a Newton telescope, and a semitransparent mirror is additionally provided in the ray path for coupling in laser light and for decoupling reflected light onto the detector.
The analysis of gaseous mixtures has acquired increasing significance in both environmental analysis and process control and monitoring technology. The requirements imposed on the measuring systems in terms of measuring sensitivity, selectivity, long-term stability, maintenance intervals and service life increase with increasing degree of automation in industry and environmental monitoring.
To make it possible to recognize a gas being released, for example, in environmental analysis and monitoring technique as fast as possible, it is desirable to cover the areas to be monitored at the closest intervals possible and over as large an area as possible. A large number of sensors, which measure locally in narrowly limited areas, and which may be connected to one another via data connections, may be used for this. Far more advantageous and effective are, however, optically imaging gas sensors, in which the light emitted is directed over large measuring sections and wherein the absorption of the reflected light represents the gas species-specific measuring effect. Such systems make it possible to obtain data on the average gas concentration in the measuring section.
The length of the measuring section is limited by the losses of light over the measuring section itself, on the one hand, and other essential restrictions arise from the losses that occur due to the optical components, for example, the concave mirror, reflector and lens systems. To reduce the losses due to scattering, the light beam emitted must reach the reflector as a light beam extending in parallel over the entire length of the measuring section. Lasers as well as laser diodes are highly suitable light sources for such measuring systems, because they have a number of advantages over thermal light sources, and these advantages make them recommendable for gas measurement: high spectral intensity, high beam quality, narrow-band spectral emission, good modulation properties, and good opto-electric efficiency. As was mentioned above, gas sensor systems with open measuring section with imaging mirror array based on a Newton telescope design are known. The drawback of the prior-art systems is that optical elements attenuating the radiation, such as mirrors and beam splitters, are located in the main ray path and thus inevitably lead to a loss of light intensity.
If a polarization beam splitter is used for beam splitting, it is necessary for the polarization of the emitted light not to be changed through the measuring section itself and the reflecting mirror, because the prerequisite for low-loss decoupling of the reflected light onto the detector is otherwise not met.
Since neither the measuring section nor the reflecting mirror leave polarization unaffected, this has the consequence that a loss of light intensity develops in the polarization beam splitter, which reduces the light intensity of the reflected decoupled light. This in turn affects the measurement of the gas concentration, which can be analyzed by the detector, in terms of resolution, because the output signal of the detector is determined, on the one hand, by the light intensity of the incident light. The overall measuring resolution of the measuring system, which can be reached on the basis of the output signal, is additionally also determined by the signal-to-noise ratio of the detector. The same applies to lens systems for beam decoupling, so that there is basically an attenuation or reduction of the quantity of light sent to the detector for the measurement and hence of the available output signal.